Polyolefins stabilized with a combination of dialkyl thiodipropionates and polyphenols



United States Patent This invention relates to stabilized polymers ofalphamonoolefins. More particularly, it relates to stabilizedcompositions and methods for stabilizing polymers of alpha-monoolefinsprepared at low temperatures and pressures.

' it is now well known that polymers of alpha-mono olefins can beprepared at low temperatures and pressures in the presence of certaincatalyst compositions which are commonly referred to as Ziegler-typecatalysts, low-pressure catalysts, or coordination catalysts. Polymersproduced by use of these catalysts are characterized by a high degree ofregularity of their molecules andare termed isotactic, stereoregular,crystallizable, crystalline, linear, low pressure, 'or Ziegler polymers.

A particularly useful low-pressure polymer is highly crystalline orcrystallizable polypropylene. It is characterized, among other things,by a high melting point, i.e., in the order'of about 170 C. and improvedtensile strength. Although there are various ways of distirr.

guishing this polypropylene from the" prior art amorphous type ofpolypropylene, 'it is convenient to characterize the I crystallinepolymer by the proportion of polymer which is insoluble inboiling'heptane, amorphous polypropylene beingsoluble therein.Low-pressure polypropylene prepared with preferred Ziegler typecatalysts contains as much as '90 to 98% heptane insoluble polymer.

Linear pplyol efins prepared with the use of Zieglertype catalystsgenerally show a gradual degradation when processed or stored.Degradation occurs particularly during exposure to elevatedtemperatures, such as may occur in milling, molding, or extrusion, andduring prolonged exposure to light, such as may occur during use. Theeffect of such degradation is to render the polymers brittle andinflexible and to lower their tensile strength,

thus reducing their usefulness for most applications.

The stabilization to which this invention relates consists intheincorporation of a combination of compounds which act as stabilizerssuppressing undesired degradation andconsequent changes in properties ofpolyolefins. Several modifications of this invention will be describedwhich provide stabilization of varying scope and among which a selectioncan be made to select stabilization which provides a particular desireddegree of protection.

Various substances which are known to give satisfactory results asstabilizers for polymers obtained by high pressure polymerization with aperoxide catalyst have been found to be unsatisfactory in the case oflow pressure, linear polyolefins. One of the major differences betweenhigh pressure polymers and low pressure polymers is that the latteralmost invariably contain significant traces of metallic catalysts.These catalyst residues can interact with some stabilizers to producecolored products or other undesirable effects.

A series of novel stabilizers for polyolefins have been recentlydescribed in U.S. 3,053,803, to laffe et al., U.S.

33%,8527 Patented June. 22, 1965 3,026,264 to Rocklin et al., U.S.3,062,895 to Martin et al., and U.S. 3,047,503 to Jaffe et al. Thestabilizers of this series are polyphenols which are characterized bythe fact that they contain at least one benzene ring on which there issubstituted at least one 3,5-dialkyl-4hydroXybenzyl radical. Thecompounds are described below in greater detail. This series of novelstabilizers includes outstandingly good stabilizers for linearpolypropylene. The preferred compounds of the series are suiiicient tostabilize polypropylene or similar polyolefins against thermaldegradation even when used alone in relatively low concentration.-

However, it is sometimes desirable to reduce the amount of thepolyphenol stabilizer below that which would provide suiiicientstabilization when used alone, e.g., toreduce the total cost ofstabilization.

It has now been found that a group of materials which are by themselvesvery ineliicient as stabilizers of polyolefins enhance the effectivenessof said polyphenyl stabi lizers to such an extent that much loweramounts of the polyphenolic stabilizers can be employed to provide adesired degree of protection.

Among the objects of the invention are: to provide novel compositions ofpolymers of alpha-monoolefins; to provide stabilized compositions ofpolymers of alphamonoolefins; to provide polymers ofalpha-monoolefinswhich have color stability and which have improved resistance toembrittlement; to provide methods for stabilizing linear or crystallinepolypropylene and other polyolefins and .to provide a method forsubstantially redpcing or preventing the tendency of polyolefins todegrade during processing operations such as are carried out during themanufacture of articles therefrom, and also during the subsequentlifetime of such articles.

The inhibitor systems of this invention are useful in linear polymers ofolefins of from 2 to 8 carbon atoms per molecule, including ethylene,butene-l, 4-methylpeutene-l and heXene-l and in crystalline copolymersof propylene with ethylene, butene-l and the like and block copolymersof alpha olefins. The term polymers of alpha-monoolefins as used in thisspecification denotes only those polymers formed from alpha-monoolefinhydrocarbon feeds, and thereby excludes polymers formed fromnon-olefinic monomers, polymers and/or copolymers. While this inventionthus applies to polymers of various alpha-monoolefins, the descriptionis directed mainly to polypropylene. It has been found thatpolypropylene is particularly readily attacked and degraded duringheating and processing and that inhibitor systems which are satisfactoryin other polyolefins may not be s is a q y n a g yls a the e he ha d btor yst m sati aptgryin s p c y n l s a y a s be satisfactory in otherlow-pressure polymers.

J The combination of stabilizing compounds which is incprporated in alinear polyple'fin to provide the novel compositions of this inventionconsists of (1) one of certain polyphenols characterized by the presenceof at least one benzene ring on which there is substituted at least one3,5-dialkyl-4-hydrpxybenzyl group; and (2) one of certain organicsulfides and polysulfides. The compounds which are useful stabilizers inaccordance with this invention are described and illustrated in furtherdetail hereinafter.

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known in prior ar. Early references are, for example, Belgian FatentsNumbers 534,792 and 538,782 to Ziegler and Ziegler et al., respectively.A great variety of suitable catalysts and conditions for their use havesince been described. Suitable catalysts can be selected from thereferences listed on pages 350361 of Linear and Stereoregular AdditionPolymers, by Gaylord and Mark, Interscience Publishers, Inc, New York,1959. Catalysts which are particularly suitable for production of highlycrystalline polypropylene, and their use, are described in U.S. Patent2,971,925 to Winkler et al.

In general, suitable coordination catalysts comprise the reactionproduct of (l) a compound of a transition metal selected from the metalsin subgroups a of groups IV, V and VI and from group VIII metals of theMendeleev Periodic Table as illustrated on page 28 of Ephraim, InorganicChemistry, 6th English edition, with a reducing compound which isgenerally a hydride or a metal-organic compound of a metal from groups1-111 of the Periodic Table.

The preferred transition metals whose compounds are used for producingvarious types of low-pressure polymers are titanium, vanadium andchromium. Titanium trichloride is a particularly preferred compound.Other suitable metals include zirconium, hafnium, tcllurium, columbium,tantalum, molybdenum, cobalt, and nickel. The preferred reducingcompounds are aluminum alkyls, particularly aluminum trialkyls anddialkyl aluminum halides. Lower alkyl groups, such as ethyl, isopropyland isobutyl are preferred. The preferred halogen element is chlorine.

The novel polyphenols to be used in compositions according to thisinvention can be generally described by Formula I wherein Ar is an arylgroup further defined below, 11 is an inte cr further defined below, andR is an alkyl radical, preferably one having up to eight carbon atoms.Since compounds having hindered phenolic groups are most satisfactory asstabilizers, it is preferred that at least one, and most preferablyboth, of the alkyl substituents R have from 3 to 8 carbon atoms and bebranched on the alpha carbon atom.

In the compounds of U.S. 3,053,803, 11 of Formula I is an integer from 1to 3 and the aryl group Ar of Formula I is preferably a phenol ring IIr-MU in which in is an integer from 1 to 3; it may also be a naphtholgroup, or it may be a polynuclear polyphenol (III) In these compounds,the value of n in Formula I is always 3. Each R is an alkyl radical,preferably one havmg from 1 to 8 carbon atoms. Methyl or ethyl groupsare suitable. The symmetrical hexa substituted benzenes,

ra es I in which alkyl groups R are in the 1, 3, and 5 position, aremost effective as stabilizers, as well as being the easiest to prepare.

In the compounds of U.S. 3,062,895, the aryl group Ar of Formula I isrepresented by the formula R111 R111 (V) Less preferred, compounds inwhich Ar represents may also be used. The value of n in formula I isalways 2 for these substituted tetraalkylbenzene compounds.

In the compounds of U.S. 3,047,503 the aryl group Ar of Formula I is apolycyclic hydrocarbon aromatic nucleus, i.e., a fused ring system whichhas more than one six-membered aromatic'ring, but which does not containan atomic bridge, a crossed valence bridge, or a free spiro union.Systems having two to four rings are preferred. The value of n inFormula I, for these compounds, is 4 when Ar is naphthalene and 6 whenAI is anthracene 0r phenanthrene, i.e., it is twice the number of fusedrings. The polycyclic aromatic nuceleus may be substituted with otherhydrocarbon radicals, particularly lower alkyl radicals.

Numerous individual compounds which meet these definitions are named andillustrated in said patents and applications. The following compoundsserve to illustrate those which are preferred for use in this invention.

Those polyhenols in which a benzene ring contains three symmetricallyspaced 3,S-di-tert-alkyl-4-hydroxybenzyl groups, the other threepositions being filled by alkyl or hydroxyl group, are generallysuperior inhibitors when used alone. When used with the described sulfurcompounds, the combinations have the advantage that only very lowconcentrations of the polyphenol are needed to provide the desiredprotection.

The most preferred phenolic compounds have a fully substituted benzenenucleus, three substituents being 3,5- di-tert-butyl-4-hydroxybenzylradicals and the remaining substituents being methyl or hydroxy groups.Especially useful results are obtained when the polyphenolic compound is1,3,5-trimethyl-2,4,6-tri(3,5-di-tert-butyl-4-hydroxylbenzyl) benzene.

Another very useful compound is1,4-bis(3,5-di-tertbutyl-4-hydroxylbenzyl)tetramethylbenzene.

Still another useful group of compounds are those in which thesubstituted benzene nucleus contains at least one hydroxyl group,particularly 2,4,6-tri(3,5-di-tert-butyl- 4 hydroxybenzyl) phenol; 3,3,5,5 tetra(3,5 di tertbutyl-4-hydroxybenzyl -4-dihydroxybiphenol;2,2-bis [3 ,5 di(3,5 di tert-butyl-4-hydroxybenzyl)-4-hydroxyphenyl]propane; and 2,4,6-tri(3,5-di-tert butyl-4-hydroxybenzyl)-3,5-dimethylphenol.

The sulfur-containing component of the stabilizing compositions of thisinvention is characterized by a linkage in which S is an integer of atleast 1, preferably 1 or 2 and most preferably 1, and the carbon atomsare members of hydrocarbon groups or of acid residues of lowercarboxylic acid esters of higher alcohols.

The hydrocarbon esters of thiodialkanoic acids which are usefulaccording to this invention may be represented by the formula:

R OOCR SR COQR wherein R and R are similar or dissimilar divalentaliphatic hydrocarbon radicals having from 1 to 6, preferably not morethan 4 and most preferably 2 carbon atoms, and R and R" are preferablyalkyl groups, but may be cycloalkyl or arylalkyl groups. R" and R arepreferably normal alkyl groups of 10 to 20 carbon Q3 atoms and mostpreferably identical groups of 12 to 18 carbon atoms each. Very goodresults are obtained with compounds which are the esters ofbetathiodipropionic acid and monohy'dric alcohols of 10to 20 carbonatoms, and particularly with the dialkyl esters. It is advisable for thehydrocarbon groups to be such that the boiling point of the compoundexceeds 250 C.

. The preferred ester for use in this invention, dilaurylbeta-thiodipropionate, is well known and is commercially available.Methods are known for preparing various esters of the same general typein which other hydrocarbon groups are present, both in the acid radicaland in the combined alcohol radical.

The hydrocarbon sulfides which are useful according to this inventionare those having from 12 to 50 carbon atoms per molecule and having thegeneral formula R S -R wherein. at is an integer and R and R representsimilar or dissimilar hydrocarbon radicals.

In two preferred modifications of this invention, the sulfur compound isa dialkyl monosulfide having at least 30 carbon atoms per molecule or adialkyl disulfide having at least 24 carbon atoms per molecule. Someimprovement is obtained with monosulfides having as few as 18 carbonatoms per molecule or disulfides having at least 12 carbon atoms permolecule, but the stabilization obtained improves drastically as thenumber of carbon atoms in the molecule increases above these minima.Sulfides having from 24 to 40 carbon atoms per molecule are particularlyslightly branched alkyls.

Useful improvements in stabilization can be obtained with a variety ofcompounds; These include dialkyl monosulfides having at least 18 carbonatoms per molecule, mixed alkyl aryl monosulfides having at least 18carbon atoms per molecule, diaryl sulfides having at least 18 carbonatoms per molecule, alkyl cycloalkyl sulfides having at least 18 carbonatoms per molecule, dicycloalkyl Another suitable group of sulfides arethe dialkyl xanthogene disulfides' (RO.CS.SS.SC.OR) in which the totalsum of carbon atoms is at least 12, e.g., dicetyl xanthogene disulfide.

Polymeric products of the type polyalkoxy alkylsulfide (R -OR S)polyalkoxy alkoxy-alkyl sulfide (-R --OR -O'R eS-) or polyhydroxyalkylsulfide (R (GH)e.S-) as described in detail in the German publishedpatent application 1,048,022 are also representative of sulfidesaccording to the invention.

The sulfides forming part of the new system of stabilizers may sometimesalso be formed in situ. Thus, for example, disulfides may be formed frommercaptans under alkaline conditions in the presence of oxygen.

The concentrations in which the stabilizers of this invention are usedare generally between 0.001 and 2% by weight of combined stabilizers.The total amount of stabilizer and the amount of each stabilizingcomponent which is used depend on several factors, such as the type anddegree of purity of the substrate, the conditions selected forincorporating stabilizer, the expected conditions during subsequenttreatment and use of the stabilized polymer, and the like. The mosteffective polyphenols are used in concentrations of at least 0.005%, andmost polyphenols are used in concentrations in the range from 0.05 to0.5%. While the sulfur compounds may be used with some effect inconcentrations as low as one-tenth of the polyphenol concentration byweight, the preferred weight ratios of sulfur compound to polyphenol arefrom 0.5 :1 to 10:1. Ratios between about 1:1 and :1 are especiallypreferred. The sulfur compounds are generally used in preferred. Thepreferred alkyl groups are normal or only I concentrations in the rangefrom 0.05 and preferably from 0.1 to 0.5% by weight. Preferred totalconcentrations of combined polyphenol and sulfur compound are in therange from 0.1 to 0.6 percent by weight for the preferred, moreeffective stabilizers. Percentages are expressed as based on the weightof polymer.

The stabilizers of this invention may be employed with many otheradditives commonly usedin polymer formulations without any adverseeffects resulting therefrom.

Typical of such additives are fillers, pigments and dyes;

The stabilizers of this invention are easily incorporated into thepolymer formulations by any known blending technique, such as dryblending-and milling. In a preferred method of adding stabilizers, atleast one of the stabilizing. components is added in the earliestpossible stage, e.g., by adding it prior to the separation of polymerfrom the reaction mixture.

If desired, the combination of two or more representatives of each ofthe two groups of stabilizers to be'used according to this invention maybe employed.

The invention is further explained by means of. the

following examples. These examples are for illustrative purposes onlyand are not to be considered a limitation of the invention.

For convenience of reference, the stabilizing compounds are identifiedin the examples by letters, as follows: A1,3,5-t1imethy12,4,6-tri(3,5-di-tert-butyl-4-hydroxybenzyl) benzeneB-1,4-bis(3,5-di-tert-buty1-4-hydroxybenzyl)tetramethyl-benzene C9,l0-bis 3,5-di-tert-butyl-4-hydroxybenzyl) anthracene D2,4,6-tri (3 5-di-tert-butyl-4-hydroxybenzyl) 3 ,5 -dimethyl phenolE.l,2,4-triethyl-3,5,6-tri(3,5-diisopropyl-4-hydroxybenzyl) benzeneFl,2-dimethyl-5-ethyl-3,4,6-tri(3-amyl-5-tert-butyl-4- hydroxybenzyl)benzene G1,4-bis (3 -tert-butyl-5-methyl-2-hydroxybenzyl)-tetramethyl-benzene H3 ,3 ,5 ,5 -tetra 3 5-di-tert-butyl-4-hydroxybenzyl) -4- dihydroxybiphenol Y I-2 ,2-bis 3,5 -di 3,5-di-tert-butyl-4ehydroxybenzyl) -4- hydroxyphenyl1propaneJ2,6-di(3,5-di-tert-butyl-4-hydroxybenzyl) phenol K-Dilaurylthiodipropionate LDi-n-hexadecyl sulfide MDin-dodecyl-thiodipropionateNDi-n-octadecyl-thiodipropionate ODi-n-eicosyl sulfidePn-Dodecyl-n-cetyl sulfide Qn-Dodecyl phenyl sulfide RDidodecyldisulfide SPolymer from dialkyl ether and hydrogen sulfide according toGerman DAS 1,048,022

EXAMPLE 1 Polypropylene employed is of. the type prepared with the useof a catalyst composed of titanium trichloride and aluminum diethylchloride in which the titanium trichloride is prepared by reactingtitanium tetrachloride with a stoichiometric equivalent of aluminumtriethyl.

The polypropylene has an intrinsic viscosity of about 3.1

(11/ g. measured at C. in decalin, and contains 0.4 phr. calciumstearate.

Testing after heating in air sulfides, alone and in combination. Resultsare given in Table 1.

TABLE 1 Polyphenol Concentra- Sulfur Concentra- Number of Compound tion,phr. Compound tion, phr. Days to Brittleness 0. 15 None 70 0.15 K 0.3106 0.15 L 0.3 130 EXAMPLE 2 A similar series of tests is carried outwith polyphenols B, C, and D in a different polypropylene substrate,namely, a commercial product having an intrinsic viscosity of 2.7,prepared with a titanium trihalide-aluminum alkyl halide catalyst andbelieved to be free of stabilizers other than those specified. Catalystresidue impurities present in the polymer are of the order of 100 ppm.alumiunm, 200 p.p.m. titanium, and 50 p.p.m. chlorine.

A further test with polyphenol A is carried out in a similarpolypropylene substrate whose intrinsic viscosity is 2.3 and whosecatalyst residue content is about 40 p.p.m. aluminum, 85 ppm. titanium,and 150 ppm. chlorine Polymer is milled for minutes at 190 C withvarious proportions of various stabilizers. Films of 5 mil (0.005 inch)thickness are pressed from the milled product at 235 C. These films areheld at 133 C. in an air-circulating oven, and periodically tested forbrittleness. Results are given in Table 2.

TABLE 2 Sulfide Compound Number of Days to Brittleness Concentra. tion,phr.

Concentra.

Iolyphenol tion, plir.

Compound None K 3 3 MM Know 0101 more 00 00 0 0. 0 NM NM CASIO EXAMPLE 3When inhibited polypropylene is exposed in an oxygen atmosphere at 135and 160 C. and oxygen uptake observed, it is found that the end of theperiod of effective ness of antioxidant is shown by a change in rate ofoxygen uptake. The length of this induction period (IR) is a measure ofantioxidant effectiveness. For a combination of two stabilizers, I andII, a synergism factor can be calculated from the data as For acombination of antioxidants A and L at a weight ratio of L:A of 3.7:1and a total concentration of 0.61% by wt., the synergism factor is foundto be 12.6. At a weight ratio of 2.6:1 and a total concentration of 0.39the synergism factor is 10.3.

EXAMPLE 4 Samples of linear polypropylene are inhibited with equal partsof several combinations of inhibitors, the total concentration ofcombined inhibitor in each sample being between 0.2 and 0.6 percent byweight of the polymer. Combinations of polyphenol B with sulfurcompounds O, P, Q, R, and S, of sulfur compound K with polyphenols E, F,G, H, I, and J and of sulfur compound L with polyphenols E, F, G, H, I,and I, when tested according to Example 1, show these combinations toprovide greater antioxidant protection than the additive effects of theseparate components.

The combinations of Examples 1-4 also provide satisfactory stabilizationof polyethylene and of polymers of high alpha-monoolefins such aspoly-4-methyl-1-butene produced with a Ziegler catalyst and of blockcopolymers of polypropylene with small amounts of other olefins.

It will be understood, and it is illustrated by the examples, that someof the polyphenolic inhibitors and some of the sulfides are superior toothers and not all combinations provide equally good results.

I claim as my invention:

1. Stabilized polypropylene composition comprising (I) solid polymerconsisting substantially of propylene produced by a low-pressurepolymerization process and containing residue of a group IV metalcompound, combined with (II) up to 1 percent by weight of an inhibitoragainst thermal degradation thereof, consisting of a synergisticcombination of (1) at least 0.05 percent by weight of a dialkylbeta-thiodipropionate wherein the alkyl group contains from 12 to 18carbon atoms, and (2) at least 0.05 percent by weight of1,3,5-trimethyl-2,4,6-tri-(3,5-di-tert butyl 4 hydroxybenzyl) benzene,

said compounds (1) and (2) being present in a ratio in the range from1:1 to 5:1, and said percentages being based on the weight ofpolypropylene. 2. Stabilized polypropylene composition comprising (I)solid polymer consisting substantially of polypropylene produced by alow-pressure polymerization process and containing residue of a group IVmetal compound, combined with (II) up to 1 percent by weight of aninhibitor against thermal degradation thereof, consisting of asynergistic combination of (1) at least 0.05 percent by weight ofdilauryl beta-thiodiproponate, and (2) at least 0.05 percent by weightof 1,3,5-trimethyl-2,4,6-tri(3,5-di-tert butyl 4 hydroxybenzyl)benzene,

said compounds (1) and (2) being present in a ratio in the range from1:1 to 5:1, and said percentages being based on the weight ofpolypropylene.

References Cited by the Examiner UNITED STATES PATENTS 9 UNITED STATESPATENTS Gribbins et a1. 26045.85 Darby et al. 26045.85 Newland et a126045.95 Newland et a1. 26045.95 Kibler et a1. 26045.75 Watchung et a126045.75 Hawkins et al. 26045.7

Hawkins et a1. 26045.7

Rocklin et al. 26045.95

Tholstrup 26045.85 Jaffe et al. 26045.95 .Taife et a1. 26045.95

Martin et al. 26045.95

10 FOREIGN PATENTS 658,288 2/ 63 Canada. 929,435 7 6/ 63 Great Britain.587,296 5/ 60 Belgium.

5 OTHER REFERENCES Sun Oil Co., 587,296 (Belgian) May 30, 1960,abstracted in Derwent Belgian Patent Reports, vol 66A, July 15, 1960.

Neureiter et al.: Synergism Between Phenols and Sulfides in IndustrialEng. Chem. Prod. Research and Development, vol. 1, N0. 4; pages 236-241,December 1962.

LEON J. BERCOVITZ, Primary Examiner.

15 DONALD E. CZAJA, Examiner.

1. STABILIZED POLYPROPYLENE COMPOSITION COMPRISING (I) SOLID POLYMERCONSISTING SUBSTANTIALLY OF PROPYLENE PRODUCED BY A LOW-PRESSUREPOLYMERIZATION PROCESS AND CONTAINING RESIDE OF A GROUP IV METALCOMPOUND, COMBINED WITH (II) UP TO 1 PERCENT BY WEIGHT OF AN INHIBITORAGAINST THERMAL DEGRADATION THEREOF, CONSISTING OF A SYNERGISTICCOMBINATION OF (1) AT LEAST 0.05 PERCENT BY WEIGHT OF A DIALKYLBETA-THIODIPROPIONATE WHEREIN THE ALKYL GROUP CONTAINS FROM 12 TO 18CARBON ATOMS, AND (2) AT LEAST 0.05 PERCENT BY WEIGHT OF1,3,5-TRIMETHYL-2,4,6-TRI-(3,5-DI-TERT - BUTYL, - 4 - HYDROXYBENZYL)BENZENE, SAID COMPOUNDS (1) AND (2) BEING PRESENT IN A RATIO IN THERANGE FROM 1:1 5:1, AND SAID PERCENTAGES BEING BASED ON THE WEIGHT OFPOLYPROPYLENE.